A power amplifier for a telecommunication system, e.g. for an RF (radio frequency)-signal transmitter in a base station or a satellite system, may have to amplify frequencies spread over a bandwidth of up to 100 MHz, due to simultaneous amplification of multiple carrier signals. Further, it is desirable that an RF amplifier has a high efficiency and dynamic range, as well as a high linearity in order to reduce the distortion.
A class D power amplifier for example, is a power amplifier in which all power devices operate in a switched (ON/OFF)-mode, and it may be switch-modulated e.g. by PWM (Pulse Width Modulation), whereby a high power efficiency can be obtained. Switched-modulation techniques normally involve a conversion of an input signal to a pulse sequence having a much higher frequency content than the input signal, a subsequent amplification of the pulse sequence, followed by filtering to remove unwanted spectral components, such as e.g. the carrier frequency harmonics or modulation noise. The resulting filtered signal corresponds to an amplified replica of the input signal. An advantage of a Class D power amplifier is the high power efficiency, which is achieved by the pulse sequence having a fixed amplitude and the switching elements being either ON or OFF, which causes a low power dissipation.
In the above-mentioned PWM, the input signal to the amplifier is modulated to give the pulses in the pulse sequence a duration, i.e. a pulse width, that is proportional to the signal amplitude. At the output from the amplifier, the pulse train is filtered with a band-pass filter to obtain its prior shape, without the presence of higher order harmonics. PWM is commonly used in power electronics, e.g. for controlling an electric engine or for power conversion, and further in audio systems, in which the introduction of PWM reduces the need for cooling of the amplifiers, as well as the size. However, in radio-frequency applications, the use of switch-modulated Class D power amplifiers is still limited due to the high switching frequencies that are needed, e.g. in the GHz-range.
Another example is a Class E amplifier, which is a high-efficient switching power amplifier that is suitable for radio-frequency signals, and consists of a single transistor driven as a switch and a passive load network.
FIG. 1 is a block diagram illustrating a conventional architecture of a switch-modulator 2, consisting of a pulse width modulator (PWM) arranged to modulate a base band input signal 1 and to present a pulse-sequence 3 forming a binary-level signal to a power amplifier 4. Since the power amplifier can be constantly driven at its maximum-efficiency operating point, the overall efficiency for the amplification of a typical amplitude-varying and phase-varying signal will be very high. Thereafter, the amplified pulse sequence 5 is filtered by a properly designed filter 6 tuned around the carrier frequency, preferably by a band pass filter for PWM, in contrast to the low pass filters for PWM commonly used for power converters or audio system, in order to filter out a correct amplified output signal 7.
In conventional PWM (Pulse Width Modulation), the amplitude of a signal is mapped onto the width of a pulse at each sample, and a single pulse is transmitted by the modulator for each incoming sample. However, e.g. in radio communication systems is required that the phase information of the signal is mapped onto some aspect of the pulse, e.g. the position, to thereby create a pulse sequence representing both the amplitude and the phase of the signal, constituting a PPM (Pulse Position Modulation) that is used together with the PWM, i.e. PWM/PPM. FIG. 2 illustrates how the amplitude of a signal 1 sampled at Ts0 is mapped onto the width, i.e. duration, of a modulated pulse 3, and the phase of the signal sample is mapped onto the position of the pulse 3 within the time interval between two samples Ts=Ts1−Ts0, i.e. the sampling interval or sampling period
The technique explained in FIG. 2 is applied in the conventional arrangement illustrated in FIG. 3, in which the amplitude-part 1 and the phase-part 9 of a baseband signal are modulated by a conventional, combined PWM/PPM 8, by which the amplitude modulates the width of a pulse and the phase modulates the position of said pulse within the sampling period of the baseband signal. FIG. 3 further shows a pulse sequence 3 created by the combined PWM/PPM 8 and onto which both the amplitude and phase of the base band signal 1 is mapped, as described above. Thereafter, the pulse sequence 3 is amplified by the power amplifier 4, and the amplified pulse sequence 5 is filtered by the band pass filter 6, resulting in an amplified base band signal 7 on the output.
Related art within the technical field is disclosed e.g. in US2004/0246060, which describes a modulator for generating a two-level signal suitable for amplification by a switching mode power amplifier, such as a Class D or a Class E amplifier.
It is further known within this technical field to combine the above-described PWM (pulse-width modulation) and PPM (pulse-position modulation) with Delta-Sigma (DS) modulation, by presenting the amplitude-part of a complex baseband signal to a 3-level Delta-Sigma-modulator, while the phase-part of the signal is presented to a DS-modulator having 8 levels. In DSM (Delta-Sigma modulation), the input signal to an amplifier is converted to a pulse sequence having a fixed pulse width and a frequency that is higher than the carrier frequency, and the average level of the bit-stream represents the input signal level. Normally, the sampling frequency fs=4·carrier frequency, such as in a fs/4 DS-modulator, resulting in a sampling frequency of e.g. 8.56 GHz in a frequency band of the 3GPP (The 3rd Generation Partnership Project). In the architecture illustrated in the block diagram in FIG. 4, the output signals from two Delta-Sigma modulators 10a, 10b are presented to a pulse-width modulating part and a pulse-position modulating part, respectively, of a combined PPW/PPM 8, to form a pulse train that has the combined characteristics of DS-modulation and PWM/PPM. The DS-modulator produces a noise-shaped spectrum, while the PWM/PPM produces a signal that consists of only those pulses that are needed to feed a switched amplifier. A signal produced by the PWM/PPM does not change as rapidly as a signal coming from a DS-modulator, which contains broadband noise with a spectral density comparable to that of the useful signal.
In the above-described FIG. 4, the amplitude-part 1 and the phase-part 9 of an input base band signal are modulated by two Delta-Sigma modulators 10a, 10b and thereafter modulated by a combined PWM/PPM 8. Since the mapping of the input amplitude onto a pulse width is a non-linear function, i.e. a sine-function, an inverse (i.e. arcsine) pre-distorter is needed to obtain a linear output, and this correction is pre-calculated in the correcting calculator 11 in the illustrated arrangement in FIG. 4. The phase information of the input signal is converted to a pulse position of the pulse sequence 3, e.g. by phase modulating an oscillator working on the intended carrier frequency, or by up-converting the base band signal to RF and extracting the phase information. Thereafter, the Delta Sigma-modulated and PW/PP-modulated signal 3 is amplified by the power amplifier 4 and filtered by the band-pass filter 6.
However, the above-described conventional arrangements, as well as related art within the technical field, involve several drawbacks. For example, a combined pulse width- and pulse position-modulation with a fixed sample period, T, may lead to so-called “wrap-around” of a pulse, which is illustrated in FIG. 5. The pulse representing the sample at Ts0 is “wrapped” within the sample period, since this first pulse cannot extend over to the next sample interval. Instead, a second pulse will be transmitted during the next interval, and this second pulse will represent the amplitude and phase of the second sample, at Ts1.
Another drawback with the combined PW/PP-modulation is the time granularity of a digitally defined pulse width and position, which restricts the achievable dynamic range due to quantization noise. At least 512 levels would be required for the width or positioning in order to reach 60-70 dB dynamic range, and this requires a clock frequency and a speed of the digital circuitry that is not achievable today. This problem can be alleviated by means of PW/PP modulation with coarse time granularity (3/8 levels) in combination with DS-modulation, but the dynamic range of the output signal is still very low, typically in the order of 40 dB for a single-carrier WCDMA (Wideband Code Division Multiple Access), and an extensive filtering is needed to reduce out of band noise to acceptable levels.
Thus, it still presents a problem to achieve a high-efficient switch-modulated power amplifier for radio-frequency signals that is capable of linear amplification of radio-frequency signals over a large bandwidth with a high dynamic range and without “wrap-around” problems.